Fuel cells increase their internal temperature in the course of power generation and generally have an internal cooling medium flow path for the flow of a cooling medium to keep the internal temperature of the fuel cells in a predetermined temperature range. In order to enhance the effect of restricting the overall temperature increase in the whole fuel cells, multiple cooling medium flow paths are provided for respective unit cells or for respective laminations of every predetermined number of unit cells and are arranged in parallel to the planes of the respective unit cells. For example, the cooling medium flow paths are formed inside gas separators as one constitutional element of the fuel cells. The gas separator is placed between each pair of adjacent unit cells for the purpose of separating a flow path of an oxidizing gas from a flow path of a fuel gas.
The operating conditions of the fuel cells, for example, the flow directions of the gases and the cooling medium inside the fuel cells, however, cause an uneven heat distribution inside the fuel cells in the course of power generation of the fuel cells. The temperature distribution inside the fuel cells is also affected by the environment surrounding the fuel cells, as well as the uneven heat distribution in the course of power generation of the fuel cells. Even in the presence of the multiple cooling medium flow paths formed inside the fuel cells to keep the operating temperature of the fuel cells in the predetermined temperature range, there may be an uneven temperature distribution inside the fuel cells. The uneven temperature distribution inside the fuel cells may worsen the power generation efficiency and the durability of the fuel cells. The even internal temperature of the fuel cells has accordingly been demanded.